An estimated 51.4 million people watched Ryan, the Republican challenger, debate Vice President Biden, a Democrat, on Thursday night, said Nielsen, the company that measures television audiences.
Four years ago, just under 70 million people watched Palin debate Biden. But that was unusual; it was a debate that had more viewers than even any presidential contest dating back to 1992.
Excluding 2008, the Biden-Ryan debate had a larger audience than any other vice presidential debate since George Bush was matched up with Geraldine Ferraro in 1984, Nielsen said.
Last week, an estimated 67 million TV viewers saw President Barack Obama in his first debate against Mitt Romney. The two men debate again next week.
More people saw the vice presidential debate on Fox News Channel than any other network. It had an estimated 10.02 million viewers, CBS had 8.31 million, ABC had 8.29 million, NBC had 7.85 million, Fox broadcasting had 5.05 million, MSNBC had 4.38 million, CNN had 4.15 million and PBS had 2 million, Nielsen said.